The polarography of glycine metal complexes and the determination of stepwise formation constants

Abstract

The polarography of metal-glycine complexes has been studied with a view to assessing its usefulness in measuring the stability of metal complexes, especially the stepwise formation constants. It is shown that the equation derived by De Ford and Hume (1951) may be transformed in a way that makes possible the calculation of both overall and stepwise stoichiometric constants from experimental data obtained with solutions of constant ionic strength. Problems in the use of maximum suppressors and buffer ions are considered. Stoichiometric stepwise stability constants for the glycine complexes of copper(II) and cadmium(II) have been determined : (i) polarographically and (ii) by the pH- titration method of Irving and Rossotti (1954a, 1954b). The values obtained by the two methods agree reasonably well. Problems of hydrolysis and precipitation arise with t8he lead(II) complexes. The zinc(II) complexes exhibit anomalous polarographic behaviour. The reasons for this behaviour are briefly discussed. Although the polarographic technique is capable of yielding values for step constants in substantial agreement with those obtained by the pH-titration technique, there is evidence that it in not so generally applicable as the latter.